206 CONDENSATION NUCLEI. 



evaporation of the iiueleiis. From recombination of these ions per- 

 sistent nncharoed nuclei will result. The facts are found to l)e in com- 

 plete accord with these considerations. 



If we produce a cloud in dust-free air by an expansion exceeding 

 1.25 after exposure to X rays, or exceeding- 1.3S in the absence of 

 ionizing agents, the drops formed, if made to evaporate by com- 

 pression, a])pear to leave behind juiclei requiring only slight ex- 

 pansion to make water condense on them. J. J. Thomson has pointed 

 out that there may be a certain size for which even uncharged drops 

 of i)uiv water may be stable in an unsaturated atmosphere. For, 

 accorcUng to the experiments of Iveinold and Riicker, the surface 

 tension of thin tilms has a mininunn for a certain thickness. There 

 may, therefore, be a certain size (somewhat smaller than that corre- 

 sponding to minimum surface tension) for which the potential energy 

 of a dro]i due to surface tension has a minimum value. Such a drojD 

 would be in equilibi'ium in vapor saturated with respect to a flat 

 surface. 



Bloch, following LangeAin, works out, in the paper already re- 

 ferred to, the theory of condensation of water vapor on ions. He 

 shows that we might expect droj)s of about 10 ///< in diameter to be 

 stable, on account of the variation in surface tension in that region, 

 l)ut we sliould not expect to meet with drojjs of which the diameter 

 was com])rised between that limit and a very low value, the equilib- 

 rium of such particles being unstable. The behavior of other sub- 

 stances than water would i)rol)ably be similar. In this way Bloch 

 explains the fact that Ave do not meet Avith ions of mobility inter- 

 mediate l)etween about 1 centimeter and one three-hundredths centi- 

 meter per second for a field of 1 xolt per centimeter. 



There are then three princii)al classes of nuclei: (1) The ions 

 l)roper, reipiiring a fourfold or sixfold supersaturation to cause water 

 to condense on theui, and having a mobility exceeding 1 centimeter 

 per second in. a field of 1 volt pvv centimeter; (2) loaded ions requir- 

 ing little or no supersaturation to make water condense on them, and 

 Inning a ui()l)ility generally less than a thousandth part of that of the 

 ions proper; (H) uncharged nuclei, resembling the second class in 

 re(|uiring little or no supersaturation in order that visible drops may 

 form ui)on them. 



I The accompanying illustrations — most of them from an article on 

 '' Nucleation dui'ing cold Aveather," by Dr. Carl Barus. which ap- 

 peared in the l^hysical Review for April, 1908 — are here appended to 

 ^liow in a gra])iiic uianucr the nuuiber of uuclei in atmospheric 

 air. Doctor liarus, who })rei)ared these charts, has for several years 

 been conducting iuAestigations on this subject, under the auspices of 

 the Smithsonian Institution, Avhich luis i)ublished several of his 

 memoirs. — Editor.] 



